56 THE ELECTRIC ARC 



The first commercial success in this direction was 

 made by Bremen who discovered the advantage of using 

 calcium fluoride. This is the basis of nearly all the com- 

 binations of salts used for flaming arcs. Other salts are 

 added to a greater or less extent to modify the kind of 

 light, or to avoid some of the disadvantages mentioned 

 above. Thus calcium phosphate is often added, especially 

 for direct-current arcs. According to Mahlke 1 cerium 

 chloride forms two-thirds of the salts used for white flaming 

 arcs, the other third being chiefly calcium salts. 



The carbon for these lamps consists of an outer cylinder 

 made of hard carbon and an inner core made of a mixture 

 of carbon and the salts to be used. The outer cylinder 

 protects the salts from too rapid consumption and gives 

 the necessary conductivity to the electrode. The inner 

 core is made of a finely divided carbon, usually lamp- 

 black, mixed with the salt, which must be in the form of 

 a powder. If the salt is not thoroughly mixed with the 

 carbon, they will produce miniature explosions and uneven 

 burning of the arc. 



According to Mahlke the core consists of 40 per cent of 

 lampblack and 60 per cent of metallic salt. There are, 

 however, few of the flaming-arc carbons now on the market 

 which have as high a percentage of salt as this. 



The resistance of these electrodes is higher than that 

 of solid carbons, and if they are long, much energy is 

 wasted in forcing the current through them. To remedy 

 this, wires are usually run through the length of the elec- 

 trodes to serve as conductors. These wires vary from one- 

 half to three-fourths of a millimeter in diameter. Zinc, 

 brass and tin have been used for this purpose. 



1 Elec. World, 57, 675; 1911. 



